Conventionally, silicate glass is used for a response glass membrane of an ion-selective electrode. Such glass is required to have such properties as small alkali error, small acid error, good response, high chemical durability, an experimental value of a potential gradient close to a theoretical value, low electrical resistance, high mechanical strength, and easiness to machine.
To improve these properties, attempts have been made to add modified metal of various types to the silicate glass. To improve the durability of the silicate glass, in particular, an attempt has been made to add an element of a cation species to the silicate glass. By way of example, it is known that a small amount of La that is a trivalent element is contained in the silicate glass so as to improve the chemical durability (including water resistance) of the silicate glass (Non-Patent Document 1).
La is filled up in a mesh structure of the glass, tightens meshes, generates a hydrated gel layer having a constant thickness, and contributes to improving the water resistance of the glass. In addition, La is a trivalent element and an ion radius of La is relatively large. As a result, an electrostatic force of a univalent anion formed by four-oxygen coordination is low. That is why La makes it difficult to respond to alkali metal, or makes it difficult to cause alkali error. For similar reasons, there is known an example of adding a lanthanoid element other than La to the silicate glass in place of La.
There is also known an attempt to add another rare-earth metal (La, Y, Nd, Ce or the like) to the silicate glass (Patent Document 1).
Non-Patent Document 1: Hisato Yoshimura et al., New Edition, Theory and Determination of pH, Maruzen Co., Ltd.
Patent Document 1: Japanese Unexamined Patent Publication No. 1990-293343
Patent Document 2: Japanese Unexamined Patent Publication No. 2006-32129